The present invention relates generally to coherent computed imaging systems, and more particularly, to adaptive filters for use with coherent computed imaging systems.
Adaptive filters are used to process complex interferograms produced by synthetic array radar (SAR) systems. The prior art relating to the present invention is to apply a spatially-invariant filter to a complex interferogram (one stage of an M-stage filter depicted in FIG. 1). The disadvantage is that these fixed filters can significantly degrade accuracy in areas of high relief or high deformation gradients characterized by drastic variations in the interferometric phase gradient. U.S. Pat. No. 5,488,374, entitled "Multi-Scale Adaptive Filter for Interferometric SAR Data" and assigned to the assignee of the present invention addresses this disadvantage but does not meet the requirement for constant independent post spacing. That requirement implies that the averaging area, or effective filter impulse response width, is constant. The present invention meets this requirement while overcoming the performance disadvantages of prior art fixed filters.
Under the best of conditions, significant filtering is required in order to make reliable interferometric phase measurements for topographic mapping and other applications of interferometric synthetic aperture radar (SAR). Conventionally, a filter with fixed impulse response width is applied to the complex interferogram. However, interferometric SAR data is extremely variable in terms of signal-to-noise ratio, intrinsic coherence, as well as the level of detail and severity of the relief present in the terrain. The optimum filter width, and even the feasible filter width, can vary from one resolution element to another in areas of high relief or high deformation gradients that result in drastic variations in phase gradients. Simulation results show that an adaptive filter can significantly improve accuracy in areas of high relief.
Accordingly, it is an objective of the present invention to provide for adaptive filters for use with coherent computed imaging systems.